When an inclining line (for example, having the angle .theta. expressed by tan .theta.=0.25) must be displayed on a display capable of displaying an image of gradational density (brightness) values, the line has conventionally been expressed as a black-and-white image as shown in FIG. 1, in which the line carries a jagged appearance in the sub-scanning direction (Y).
To smooth such a jagged border line, conventionally so-called a twin beam method is put to practical use. In the method, a line expressed in black-and-white is accompanied by a sub-line expressed in the middle level density (brightness) obtained from a computation when they are written into an internal memory of a display.
FIG. 2 shows an example of a black-and-white image having the same inclination as the line shown in FIG. 1 displayed on a display by means of the twin beam method.
Assuming that each of beam dots a, b, c, d . . . composing the inclining line (called a "main line" hereinafter) shown in FIG. 1 has a brightness level of 5 (this number means the highest brightness), in the twin beam method shown in FIG. 2, some of the brightness of each beam dot composing the main line is distributed to corresponding one of accompanying sub-beam dots a', b', c', d' . . . depending on the inclination and the position of each beam dot. In this, the brightness level .sigma. of a main beam dot and the brightness level .sigma.' of its accompanying sub-beam dot amounts to the brightness level of 5=.sigma.+.sigma.' (for example, if .sigma.=4, then .sigma.'=1) to smoothe the jagged appearance of the inclining line. The number deposited in each beam dot in FIG. 2 indicates the brightness level thereof.
In the abovementioned twin beam method, the processing for image-reproduction cannot be carried out in realtime, because the computation for determining proper brightness for all the main and sub-beam dots composing a black-and-white image is rather complicated and time-consuming.
In another aspect, in a color scanner for reproducing images on a photosensitive film, characters or drawing lines are usually recorded on the scale of a drawing pixel which corresponds to one severalth of a pictorial pixel (this is called a "high-resolution recording process"). An apparatus for practicing such a high-resolution recording process can produce a reproduction image containing rather smoothed border lines, however, the effect thereof is brought only to designated border lines and to black-and-white image components, not to border lines included in the other pictorial components (such as continuous tone images).